1. Field of the Invention
This invention relates generally to the selective scaling, such as magnification and reduction, of still video images, the information for which is stored digitally in a single frame buffer memory; and more particularly concerns the method of determining the picture element values of the scaled image as a function of their positions relative to their counterparts in the unscaled image.
2. Description of Prior Art
Systems exist which can selectively scale still video images. One such system employs a video memory having considerably larger capacity in terms of display locations than the display has picture elements (pels). In the unscaled state, such systems display with adjacent pels data stored in every n'th memory element, rather than adjacent memory elements. Upon scaling (in this example by magnification), intervening, previously undisplayed memory elements are called into use, n growing successively smaller with each increase in magnification, until it reaches one, and adjacent pels display adjacent memory elements, covering a smaller portion of the subject. The opposite occurs in an image reduction.
Systems also exist which scale without using additional image data stored in memory. Typically, such systems create the data needed for magnification by replicating or averaging the values of the previously adjacent pels they will be separating.
Systems incorporating more memory than display capacity incur substantially higher costs for increased memory space and increased processing time needed to access and manipulate the image data, generally on a bit-by-bit basis. A further disadvantage is that the larger pel array may not be available. Very often the image was captured by a video camera connected to the same frame buffer memory which is storing and displaying the image. Therefore, all available pels are already being displayed and any scaling must be done solely with those pels. Those systems which create the magnification data y replication or averaging of "nearest neighbor" data incur certain image inaccuracies and artifacts. These inaccuracies are particularly counterproductive to the scaling of images consisting largely of narrow curvilinear strokes, such as handwriting and drawing. Averaging systems are at a particular disadvantage when the image data is only one bit per pel. Many display systems, including most facsimile systems and many computer displays are limited to two values (attributes) per pel. These are typically referred to as black/white systems, although many displays and screens are actually orange and black or green and black. Since these systems lack "grey", the average of two pels will have to be rounded to one of the two values--not a true average at all.